Recently, the demand for an automatically operated lathe, provided with a turret-type tool rest to which a plurality of tools can be attached, has increased in order to realize an automatic and high-speed processing or machining operation and to correspond to multi-item production of small quantities. It should be noted that, in this specification, the term "automatically operated lathe" means any lathe, such as an NC lathe, that can automatically perform a lathe turning process.
The turret-type tool rest is structured in such a manner as to rotate a turret mounted on a tool rest body so as to automatically index a plurality of (generally, five to twelve) indexed positions. The turret is provided at the outer circumferential surface or the axial end surface thereof with a plurality of tool-attachment portions corresponding to the indexed positions. various tools, such as a single point tool, a drill, etc., can be attached to each of the tool-attachment portions, and an appropriate tool which meets machining particulars can be automatically selected by the indexing rotation of the turret so as to be located at a predetermined operative position on the automatically operated lathe. It should be noted that, in this specification, the term "axial" means a direction along the rotational or central axis of an object.
In a condition where a selected tool performs a turning or drilling process on a workpiece, it is required to firmly fix the turret at a selected indexed position on the tool rest body. Accordingly, a pair of coupling elements which can be engaged with and disengaged from each other (or clamp/unclamp actions) are generally provided between the tool rest body and the turret. The engagement or disengagement of the pair of coupling elements makes the turret freely rotate in an indexing manner or locates and fixes the turret at a predetermined indexed position.
In general, as a coupling unit including the pair of coupling elements, a construction (referred to, for example, as a CURVIC coupling) has been used, wherein coupling elements having crown-gear shapes, each provided with a large number of teeth on an axial end surface, are engaged with each other.
Normally, in this type of coupling unit, one coupling element provided in a turret side is linearly shifted in an axial direction together with the turret, relative to another coupling element provided in a tool rest body side, so as to carry out the clamp/unclamp actions. Accordingly, in order to rotate the turret in an indexing manner, the turret is first moved in the axial direction relative to the tool rest body so as to release the interengagement of the coupling elements (an unclamp action), and the indexing rotation is performed in this state. Then, after the next tool is indexed, the turret is again moved in the axial direction so as to have the coupling elements interengaged (a clamp action), whereby the turret is fixedly held at the selected indexed position.
The indexing rotation of the turret is generally performed, after a machining step by a certain tool has been finished, so as to subsequently carry out a machining step by the other tool, as one step in the sequential steps of an automatic machining operation. To this end, during the indexing rotation, an unfinished workpiece and a spindle stock for supporting the workpiece are disposed near the turret.
Consequently, if the indexing rotation of the turret is performed while leaving the situation where the machining step by a certain tool has been finished, disadvantage may be caused wherein the turret or the tool attached to the turret interferes or collides with the spindle stock or the workpiece, due to the axial movement of the turret for releasing the coupling elements.
In order to solve the above problem, when the rotational indexing operation of the turret is incorporated into a processing or machining program, in a conventional automatically operated lathe, the machining program is written in such a way that the rotational indexing operation involving the clamp/unclamp actions is performed after the turret-type tool rest or the tool is shifted relative to the spindle stock or the workpiece into a location (a retreated position or a tool home position) where the interference due to the axial movement of the turret is avoidable, and that the turret-type tool rest is returned back to the operative position after the rotational indexing operation has been completed.
Incidentally, the machining program of the automatically operated lathe is generally written by a user in a manual operation so as to correspond to the types or the machining particulars of workpieces. The user normally writes the machining program which should be entered into the automatically operated lathe, while referring to the design drawings of a product.
However, the axial movement of the turret, accompanying the rotational indexing operation, is a movement resulted from the construction of the automatically operated lathe and, therefore, cannot be recognized from the design drawings of a product. Consequently, it has been troublesome for the user to make the machining program in such a manner as to always consider the above-described axial movement of the turret. Also, it is considered that an inappropriate machining program, which dose not take the axial movement of the turret accompanying the rotational indexing operation into consideration (i.e., which does not include the retreat and return movements of the turret-type tool rest), is carelessly built. If such inappropriate machining program is entered into the automatically operated lathe, there is a possibility in that the turret or the tool will interfere with the spindle stock or the workpiece to damage them during the rotational indexing operation.
In the case where the turret-type tool rest is operated for the above-described retreat and return movements whenever the turret is rotated in the indexing manner, the machining time is extended by at least a time spent for the retreat and return movements. In order to reduce the unnecessary extension of the machining time as much as possible, it is conceivable that the retreat and return movements of the turret-type tool rest is started simultaneously with the activation of the axial movement, i.e., the unclamp/clamp actions, of the turret, but, in this case, the problem of interference cannot be fully solved. Also, in the case where the mutual interference between the turret or the tool and the spindle stock or the workpiece does not occur even when the indexing rotation is performed, the retreat movement of the turret-type tool rest is regarded essentially as an unnecessary movement if the axial movement of the turret is not required.